Primal and subprimal meat cuts are large cuts of meat. They are smaller than a side of beef, for example, but larger than the ultimate cut which is sold at retail to the consumer. A primal cut comprises the entire section of a side of beef, such as the rib section or the rump roast section, while a subprimal cut comprises only a portion of such a section. Primal and subprimal cuts are prepared at the slaughter house and are then shipped to a retail meat store, or to an institution such as a hospital, hotel or restaurant, where they are butchered into smaller cuts of meat suitable for the individual consumer.
When primal and subprimal meat cuts, such as roast or rib sections, are prepared at the slaughter house, they are usually packaged in such a way that air (i.e., oxygen) is prevented from contacting the meat and moisture is prevented from leaving the meat. This is done in order to minimize spoilage and discoloration during shipping and handling. One desirable way to package primal and subprimal meat cuts so as to protect them from contact with air and from moisture loss is to shrink package them with a packaging material that has good oxygen and moisture vapor barrier properties. One such shrink packaging material that has good oxygen and moisture vapor barrier properties is polyvinylidene chloride film. (Polyvinylidene chloride copolymers are commonly referred to as PVDC.)
While polyvinylidene chloride film has excellent barrier properties, in actual practice, when polyvinylidene chloride is used as a monolayer film, it must be plasticized in order for the film to have adequate abrasion resistance and flexibility at storage temperatures of, for example, 30.degree. to 50.degree. F. Unfortunately, the addition of plasticizer sufficient to provide the requisite low temperature properties to the PVDC monolayer film has a significant adverse effect on the barrier properties of the film. While increasing the thickness of the film from the conventional thickness of 1.5 to 2.0 mils, to 5 mils or more, for instance, would improve the barrier properties of the film, it would be economically undesirable to use a monolayer film of polyvinylidene chloride having a thickness of 5 or more mils. Also, if such thick films were employed, bags made from the film would be difficult to gather and clip closed at the open end.
One approach to the provision of a film having barrier properties which are better than those of the 1.5 to 2.0 mil monolayer polyvinylidene chloride film previously used for shrink packaging primal and subprimal cuts, is to employ a multilayer film, one layer of which is polyvinylidene chloride having a minimum amount of plasticizer. The other layer or layers of such multilayer films are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in the polyvinylidene chloride layer containing little or no plasticizer.
In providing such a film, however, it must be recognized that good oxygen and moisture vapor barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging primal and subprimal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient for the film to heat-shrink within a specified range of percentages, e.g., from about 15 to 60 percent at about 90.degree. C., in both the machine and the transverse directions. (Conventionally, the term "MD" refers to machine direction and the term "TD" refers to transverse direction.) The film must be heat sealable in order to be able to fabricate bags from the film and in order to heat seal the open ends of the fabricated bags after insertion of the meat product. The heat sealed seams of the bags must not pull apart during the heat shrinking operation, and the film must resist puncturing by sharp bone edges during the heat shrinking operation. Also, there must be adequate adhesion between the several layers of the film so that delamination does not occur, either during the heat shrinking operation or during exposure of the film to the relatively high temperatures that may be reached during shipping and storage of the film in the summertime.
One recent development in the packaging of primal and subprimal meat cuts is the use of a machine system wherein the multilayer bag containing the primal or subprimal meat cut is placed in an evacuation chamber wherein all air is evacuated from the chamber and from the bag contained therein. The mouth of the evacuated bag, which has been placed in a lay-flat condition upon the back-up bar or anvil of an impulse heat sealing unit contained within the evacuation chamber, is then sever-sealed within the evacuated chamber by the immediate action of the heat sealing unit. The sealing bar of the heat sealing unit comes down from the top of the evacuation, chamber and pins the bag mouth tightly to the back-up bar so that the film is immobilized during sealing. The sealing bar contains an impulse heat sealing element and a severing knife. The knife severs excess multilayer film from the bag mouth which is being sealed by the impulse sealing element. A specified voltage is applied to the impulse sealing element for a time sufficient to heat and thereby fuse the two inside layers of the flattened bag mouth so that the evacuated bag is sealed tightly without damage to the seal or to the adjacent body of the evacuated bag. Such a machine system is taught, for example, in U.S. Pat. No. 4,541,224 to Mungai.
Closing the mouth of an evacuated bag in this manner eliminates the problems encountered with clipping the bag mouth closed. The clip-closing of the bag mouth requires the tight gathering of the bag mouth into compressed folds which are locked in place by the application of a plastic or metal clip. The gathered folds are not always air tight, so that air will often leak thereafter into the evacuated bag through the folds and cause spoilage of the meat contained therein. Sometimes such leakage can be eliminated by adjusting the crimping pressure of the clipping device so that the clip can more tightly grip the gathered bag mouth, but such a move is not always successful. Often, the clip will hold the gathered bag mouth so tightly that clip damage occurs. Clip damage arises when the clip is so tight that it severs the multilayer plastic film in one or more of the gathered folds, thereby allowing air to leak into the evacuated bag through the severed folds.
Closing the mouth of the evacuated bag by the heat sealing technique is not always troublefree either, however. Sometimes the inside layers of the multilayer film in the bag mouth do not completely and tightly fuse together. At other times the multilayer film in the bag mouth will overheat and damage will occur. Such damage includes the melting of the film so that it flows out from under the sealing bar to leave a defective seal. It also includes charring of the film. Damage to the seal in the bag mouth in this manner is commonly referred to as "burn-through". In any event, defective seals caused by inadequate fusion of the bag mouth or by burn-through allow air to leak into the evacuated bag and thereby spoil the meat contained therein.
In order to avoid such sealing defects, it is not only important that the heat sealing unit be in proper adjustment, but it is also important that the sealable outer layer of the multilayer film, which is fabricated into the bag, have heat sealing characteristics which are compatible with the heat sealing operation. Moreover, once the heat seal has been made, the evacuated and sealed bag is sent to a shrink tunnel wherein it is exposed to a temperature, conventionally about 90.degree. C., which is sufficient to shrink the multilayer bag down tightly onto the enclosed meat product in order to present an attractive finished package for shipment to the purchaser. It is essential that the heat seals not delaminate during the heat shrinking operation. That is to say, the shrink tension in the multilayer film must not be sufficient to cause the fused inner plies of the heat seal to peel away from each other and thereby allow air to seep into the bag.
The prior art teaches the fabrication of films having improved physical characteristics by the cross-linking of the heat sealable layer. It is also known that the heat sealing characteristics of a film may be affected by adjustment of the monomer content of the thermoplastic copolymer. The prior art also teaches films having improved heat sealing characteristics by the addition of specified novel chemical constituents to the heat sealing layer.
Brax et al teach in U.S. Pat. No. 3,741,253 a multilayer film having a first outer layer of a cross-linked copolymer of ethylene and vinyl acetate, a core layer of an oxygen barrier film (PVDC), and a second outer layer of ethylene vinyl acetate copolymer which is not cross-linked. The cross-linking is achieved by irradiation of only the first outer layer of the film at a dosage of from 2 to 15 megarads. This cross-linked ethylene vinyl acetate copolymer layer provides the heat sealing layer when the film is fabricated into a bag. Brax et al teach that the radiation dosage will effect the physical properties of the irradiated ethylene vinyl acetate layer and, further, that the vinyl acetate content of the ethylene vinyl acetate copolymer will also effect physical properties. In particular, Brax et al teach that seals delaminate when the vinyl acetate content of the copolymer exceeds 18 wt. %.
In U.S. Pat. No. 4,448,792 Schirmer teaches a cook-in shrink bag fabricated from a multilayer film having a first heat sealing and food contacting layer composed of propylene homopolymer or copolymer; a second heat shrinkable layer composed of a blend of propylene homopolymer or copolymer and butylene homopolymer or copolymer; a third adhesive layer composed of irradiatively cross-linkable ethylene copolymer: a fourth barrier layer comprising vinylidene chloride copolymer; a fifth adhesive layer of irradiatively cross-linkable ethylene copolymer: and a sixth optical clarity layer comprising propylene homopolymer or copolymer; wherein the entire six layer film has been irradiated either before or after the multilayer film has been biaxially stretched. Among the several key benefits enumerated by Schirmer, the completely irradiated six layer film provides for heat seals having high temperature resistance, while at the same time being acceptable for food contact in terms of minimum levels of extractables. Cook-in bags made of this multilayer film will maintain seal integrity when submerged in water at 80.degree. C. for 12 hours.
Schirmer also teaches in U.S. Pat. No. 3,808,304 that the heat sealability of oriented polypropylene may be improved by blending into the polypropylene a specified percentage of polybutene-1. Polypropylene is only slightly sealable at temperatures below its melting point, and Schirmer discovered that blends of polypopylene and polybutene-1 could be heat sealed at temperatures below 220.degree. F. and even as low as 160.degree. F. Neither polymer by itself can be heat sealed at these temperatures. The strength of the heat seal was found to depend on the sealing temperature and the blend ratio. No irradiation or cross-linking of the film is required.
U.S. Pat. No. 4,502,263 to Crass et al covers a sealable transparent polyolefinic multilayer film comprising a base layer of polypropylene polymer and at least one sealable layer. The sealable layer contains a propylene copolymer, a low-molecular weight resin, a propylene homopolymer, and a polydiorganosiloxane. The film has a wide sealing range, low sealing temperature, high gloss, improved scratch resistance, and good reliability of running in high-speed packaging machines.
U.S. Pat. No. 4,348,457 to Rosenthal et al disclosed a sealable multilayer film comprising a polypropylene layer and at least one sealable layer comprising an ethylene homopolymer or copolymer which contains an additive combination of a long-chain amine, a thermoplastic polymer incompatible with the ethylene sealing polymer, and a polydialkyl siloxane. There is no specific teaching of the cross-linking of any layer. The film has improved sliding properties and has very good processability in high speed packaging machines. It is especially suitable for wrapping individual packs of cigarettes.
With this then being the state of the art, it is an object of the present invention to provide a multilayer film having improved heat sealability.
It is another object of the present invention to provide a heat shrinkable multilayer film, suitable for use in packaging primal and subprimal meat cuts, having improved heat sealability.
It is a further object of the present invention to provide a multilayer film having improved heat sealability under conditions experienced in an impulse heat sealing device.
It is a still further object of the present invention to provide a heat shrinkable multilayer film, suitable for use in packaging primal and subprimal meat cuts, having improved heat sealability under conditions experienced in an impulse heat sealing device.